1. Field of the Invention
The present invention relates to a lead frame and a semiconductor device having the same as well as a method of resin-molding the same.
All of patents, patent applications, patent publications, scientific articles and the like, which will hereinafter be cited or identified in the present application, will, hereby, be incorporated by references in their entirety in order to describe more fully the state of the art, to which the present invention pertains.
2. Description of the Related Art
In recent years, the importance of the high density packaging technique for realizing an ultra-high density integration of the semiconductor device has been on the increase. In case that the semiconductor chip has a single transistor such as a power transistor, it is important to mounting a heat radiator onto the semiconductor device, even a possible reduction of the cost of the semiconductor device is strongly required. In order to satisfy both requirements for the heat radiation and the cost reduction, a “down-set” was proposed, wherein each die pad of the lead frame is depressed down or set down, so that the die pad has a lower level than the other part of the lead frame, and the semiconductor chip is thus mounted on this down-set die pad. The semiconductor device mounted on the down-set die pad is then resin-molded, wherein a bottom surface of the down-set die pad is exposed or shown from the bottom of the molding resin, so that the down set-die pad serves as a heat radiator or a heat radiation channel.
The down-set process for the die pad causes a variety of mechanical stress to the die pad, suspension pins and frame portions. In case, this mechanical stress may cause a disconnection of one or more suspension pins and/or any undesirable deformation of the frame portions.
Japanese laid-open patent publication No. 5-315499 discloses a conventional lead frame structure designed for avoiding any undesired deformation due to the down-set process. FIG. 1 is a schematic fragmentary plan view of a conventional lead frame structure designed for avoiding any undesired deformation due to the down-set process disclosed in Japanese laid-open patent publication No. 5-315499. A lead frame 101 has double frames 102 extending in parallel to each other. A plurality of die pad 103 is aligned between the double frames 102 and in a direction parallel to the extension direction of the double frames 102. Each the die pad 103 is mechanically connected through a pair of suspension pins 104 to the frames 102. Namely, the suspension pins 104 are thus positioned inside of the frames 102 and opposite outsides of the die pad 103 so as to provide a mechanical connection between the each die pad 104 and the frames 102. The each die pad 103 is also surrounded by a set of plural leads 105 which provide electrical connections through bonding wires to a semiconductor chip not shown which is mounted on the die pad 103.
The each semiconductor chip is mounted on the each die pad 103, where the die pad 103 is depressed by a die, whereby the level of the die pad 103 becomes lower than the frames 102. This process for level-down of the die pad 103 is so called to as “down-set”. This down-set of the die pad 103 causes a certain increase in a distance in three-dimensional space between the die pad 103 and the frames 102. This amount of increase of the distance depends on the amount of the level down.
In accordance with this conventional technique, the suspension pins 104 are modified for responding to variation in the distance between the die pad 103 and the frames 102 upon the down-set process. FIG. 2 is a fragmentary plan view of a pair of the modified suspension pins for providing a mechanical connection with a flexibility in distance between the die pad and the frames included in the lead frame of FIG. 1. Each of the paired suspension pins 104 has a zigzag shaped portion 104a in plan view. This zigzag shaped portion provides a certain mechanical flexibility which allows a limited variation in the distance between the die pad 103 and the frames 102 upon the down-set process. This mechanical flexibility absorbs or relax a certain mechanical stress applied to the suspension pins 104 due to variation in the distance between the die pad 103 and the frames 102 upon the down-set process. This mechanical flexibility also makes it easy to accomplish the down-set process. For those reasons, the zigzag shaped portion may also serve as a mechanical stress absorption portion.
FIG. 3 is a cross sectional elevation view of the down-set die pad connected through the modified suspension pins to the frames in FIG. 1. In the down-set process, the die pad 103 is depressed by the press of the die and becomes level-down by a depth “D” form the frames 102, while having a horizontal distance “T” remain unchanged, wherein the zigzag shaped portions 104a of the suspension pins 104 are elastically deformed to show a stretch or an extension by an increase in the distance between the die pad 103 and the frames 102. After the die is released or removed from the state of pressing the die pad 103, the zigzag shaped portions 104a still remain stretched to provide substantially no mechanical stress to the frames 102. Namely, the zigzag shaped portions 104a make the frames 102 free from any undesired deformation in connection of the down-set process. The zigzag shaped portions 104a also make the suspension pins 104 free from a possible disconnection involved in the down-set process.
Consequently, the zigzag shaped portions 104a provide such both a desired mechanical flexibility and a mechanical stress absorption as to allow the down set process to make the die pad 103 become level-down from the frames 102 by a depth “D”, while having the horizontal distance “T” remain unchanged, without causing any undesired deformation of the frames 102 and any disconnection of the suspension pins 104. As shown in FIG. 3, he horizontal distance “T” is defined in a horizontal direction perpendicular to the longitudinal direction, along which the double frames 102 extend.
Japanese laid-open patent publication No. 5-267539 also discloses another conventional lead frame structure designed for avoiding any undesired deformation due to the down-set process. The disclosed technical concept in connection with the counter-measure to the down-set process is substantially equivalent to the above-described conventional technique. A lead frame has a modified support bar which acts as a suspension pin for supporting a stage which acts as a die pad. The modified support bar has a stretchable s stress absorber portion which may exhibit substantially similar function as the above-described zigzag shaped portions, but which are structurally different from them. The stretchable s stress absorber portion comprises an annular portion connected to the support bar and a cross sectional portion.
Returning back to the issue of heat radiation, a heat radiation technique is important for mounting a power transistor such as MOSFET, MESFET or bipolar transistor, particularly for a vertical type MOSFET with a larger current in operation.
In parallel to the above requirement for improvement in the heat radiation structure, a possible shrinking technique of the semiconductor device or a possible increase in the packaging density of the power transistors with the lead frame are also important in view of a possible cost reduction and a possible size-down of an electron device, on which the semiconductor devices are mounted.
The present inventor confirmed the following facts. A highly stretchable material for the suspension pin is effective to avoid a possible disconnection of the suspension pin in the down-set process, but which is insufficient in hardness for the lead frame. Namely, the realization of the practically using lead frame needs sufficient hardness not only to the frames but also the suspension pins. If the suspension pin has an increased hardness, then this may cause another problem with undesirable limitation to the necessary stretchability and may allow a frequency of disconnection of the suspension pins. For those reasons, simple dependence upon selection of any stretchable material for the suspension pins are still engaged with any problems described above.
Further, as shown in FIGS. 2 and 3, the above-described conventional modified suspension pin 104 has the zigzag-shaped stretchable portion 104a which gives rise to a certain increase in the length size of the suspension pin 104. The horizontal size “T” should be sufficiently large with reference to the depth “D”. Ensuring the sufficiently large horizontal size “T” of the suspension pin 104 makes it difficult to realize a desired increase in the packaging density.
As shown in FIG. 2, the size of the die pad is defined by subtraction of a double of the horizontal size of the suspension pin 104 from the horizontal distance between the paired frames 102. Increase in the horizontal size “T” of the suspension pin 104, while fixing the horizontal distance between the paired frames 102 would give rise to a certain decrease in the size of the die pad which provides a possible occupied area for the semiconductor device to be mounted on the die pad 103. The decrease in the size of the die pad results in a decrease in the packaging density of the semiconductor device. The decrease in the packaging density make it difficult to realize a desired shrinkage of the semiconductor device. Further the increase in the size of the lead frame makes it difficult to achieve an appreciable cost reduction.
A further important issue for manufacturing the semiconductor device is how to improve the return on investment. It will be preferable as an example for improving the return on investment that the same dies are commonly used for semiconductor packages of different types, for example, down-set free package, down-set packages with various depths in down set. In accordance with the above-described prior art, however, the necessary minimum length of the suspension pin would depend on the down-set, and also the depth of the down-set. For this reason, the prior art does not allow the desired common use of the same dies for a down-set variety of the semiconductor packages the prior art is unsuitable for improving the return on investment.
In the above circumstances, the development of a novel lead frame structure free from the above problems is desirable.